1. Field of the Invention
The present invention relates to a ridge-waveguide semiconductor laser device used for optical communications and so on, which has improved temperature characteristics and high-speed operating characteristics.
2. Background Art
In recent years, optical fiber networks have been popularized rapidly and the use of communication semiconductor laser devices spreads from trunk lines to subscriber loops. Further, in accordance with rapid popularization of the Internet and development of computer networks, the necessity of higher speed LANs (Local Area Networks) using optical fibers has increased. The semiconductor laser device for data communication, that is the key device of optical fiber networks, is required to have better temperature characteristics and to operate at higher speed. The semiconductor laser device that operates at a high speed and at a high temperature without adding a special cooling device is indispensable, to reduce the cost of optical communication equipments using the device.
FIG. 6 and FIG. 7 show the conventional ridge-waveguide semiconductor laser device. FIG. 6 is a perspective view showing the conventional ridge-waveguide semiconductor laser device that employs an n-type semiconductor substrate. FIG. 7 is a perspective view showing the conventional ridge-waveguide semiconductor laser device that employs a p-type semiconductor substrate.
The ridge-waveguide semiconductor laser device shown in FIG. 6 is manufactured as follows. An n-type InP cladding layer 2n is deposited on the upper surface of a semiconductor substrate 1n, such as n-type InP substrate. Further, on the upper surface of the n-type InP cladding layer 2n, an AlGaInAs multi-quantum well active layer 3 is deposited. On the upper surface of the active layer 3, a p-type InP cladding layer 5p is deposited and formed to have three protruded ridges therein. On the upper surface of this cladding layer 5p, a p-type electrode layer 7p is deposited with a p type InGaAs contact layer 6p intercalated therebetween. Among the three protruded ridges, the central protruded ridge constitutes a ridge-waveguide 9 with an active region 10 formed in the active layer 3 located under the central protruded ridge. In addition, on the undersurface of the n-type semiconductor substrate 1n, an n-type electrode layer 8n is deposited.
Generally, a series resistance Rd is one of the elements that governs the temperature characteristics of the laser device. As this series resistance Rd becomes small, the generated Joule heat decreases, that leads to better temperature characteristics. Further, the series resistance Rd is also one element that determines the operating speed of the semiconductor laser device. The smaller the series resistance Rd is, the smaller the RC time constant becomes, that leads to better frequency response. Therefore, the series resistance Rd is preferably as small as possible in order to improve the operating speed of the device.
Since the conventional ridge-waveguide semiconductor laser device in FIG. 6 is constructed using the n-type semiconductor substrate 1n, the cladding layer 5p constituting the ridge-waveguide 9 is formed inevitably using a p-type semiconductor, for example, p-type InP or p-type InGaAs. However, as the majority carrier of the p-type semiconductors is holes, its resistivity is about ten times higher than the resistivity of n-type semiconductors. Therefore, the conventional semiconductor laser device shown in FIG. 6 has a larger resistance in the portion of the ridge-waveguide 9 close to the active region 10 above the n-type semiconductor substrate in, thus the series resistance Rd of the device is large. This large resistance causes the heat generation and the large RC time constant, which offers a disadvantage in improving the temperature characteristics and the operating speed characteristics.
FIG. 7 shows a conventional ridge-waveguide semiconductor laser device that aims to solve this problem. This device is constructed using a p-type semiconductor substrate 1p, and is manufactured as follows. Ap-type InP cladding layer 2p is deposited on the upper surface of the p-type semiconductor substrate 1p such as InP. Further the AlGaInAs multi-quantum well active layer 3 is deposited thereon. On the upper surface of this active layer 3, an n-type InP cladding layer 5n is deposited and formed to have three protruded ridges. On the upper surface of this cladding layer 5n, an n-type electrode layer 7n is deposited with an n-type InGaAs contact layer 6n intercalated therebetween. Among three protruded ridges, the central protruded ridge constitutes the ridge-waveguide 9, and the active region 10 is formed in the active layer 3 under the central protruded ridge. In addition, on the undersurface of the p-type semiconductor substrate 1p, a p-type electrode layer 8p is deposited.
In the ridge-waveguide semiconductor laser device including the p-type semiconductor substrate 1p shown in FIG. 7, the ridge-waveguide 9 is composed of the n-type InP cladding layer 5n and the n-type InGaAs contact layer 6n. Thus the series resistance Rd for a current passing through the ridge-waveguide 9 should be reduced. Hence the temperature characteristics and the operating speed characteristics are expected to be improved. However, in reality, the temperature characteristics are not sufficiently improved. In the device, a comparatively thick portion of the n-type InP cladding layer 5n remains between the ridge-waveguide 9 and the active region 10. This thick portion should be left for avoiding an excess of etching in the etching process to shape three protruded ridges in the InP cladding layer 5n. This comparatively thick portion of the cladding layer 5n left between the ridge-waveguide 9 and the active region 10 gives rise to a large leakage current passing through that portion. This large leakage current causes an oscillation threshold current to increase, and accordingly leads to larger heat generation. Therefore, the temperature characteristics deteriorate.
This invention proposes a ridge-waveguide semiconductor laser device using the p-type semiconductor substrate that achieves improvement in the temperature characteristics and the operating speed characteristics.
According to the present invention a ridge-waveguide semiconductor laser device includes a p-type semiconductor substrate, a p-type cladding layer, a quantum well active layer, an n-type thin first cladding layer, an n-type thick second cladding layer which is made of different semiconductor materials from the first cladding layer, and two trenches formed in the second cladding layer that shape a ridge-waveguide between the two trenches. Each of the trenches reaches to or reaches in vicinity to the surface of the first cladding layer.
Other and further objects, features and advantages of the invention will appear more fully from the following description.